Membranes, esp. polymeric membranes, provide an economical method of presenting large surfaces to electromagnetic, acoustic, or other energy, for the purpose of absorbing, reflecting, focusing, or other manipulation of this energy.
Flint (U.S. Pat. No. 2,300,251, 1941) describes fabricating a lens by using two transparent membranes with a clear fluid between. Focal adjustment is made by mechanically adjusting the frame, or by varying the fluid pressure between the membranes, or diaphragms.
Membranes have also enabled the design of inexpensive mirrors when a membrane is coated with a reflective coating, and then are stretched over a suitable frame.
Martinez (U.S. Pat. Nos. 3,687,524, 3,757,479, and 3,877,139 1972-1975) described a flat “glassless” mirror stretched over a sheet metal “pan”.
Additionally, the shaping of such stretched reflective membrane mirrors into a concave or convex lens shape enables the mirror surface to now become a reflective lens with an approximate spherical/parabolic shape useful for radiating, receiving, reflecting or focusing electromagnetic radiation or acoustic waves. A concave shape may be established by applying a vacuum within the sealed chamber formed by the membrane and it's supporting frame. A convex shape can be established by applying a positive pressure within the sealed chamber. For space-based applications where near vacuum ambient conditions exist, a second clear membrane may be placed over the reflective membrane, and positive pressure introduced between the two membranes to induce a concave shape into the reflective inner membrane.
Such designs have been suggested for use in solar concentrating dishes, radio antennas and also for imaging applications such as telescopes and holographic projection.
Prior art with reference to utilizing a stretched membrane material as a lens, or reflective lens include:
Pajes (U.S. Pat. No. 2,952,189, 1960) designed a drum which is evacuated to induce a concave shape.
Kopitko (U.S. Pat. No. 3,031,928, 1962) described a dual diaphragm system and a controlling method to maintain curvature.
Bochmann (U.S. Pat. No. 3,610,738, 1971) depicted an arrangement whereby a diaphragm was pulled by a cam to induce a vacuum, and hence concave shape.
Brantley, Jr. et al, (U.S. Pat. No. 4,033,676 1977) depicted two hoops separated by vertical strut members. The two membranes form a cavity inside the circumference, and membranes also form a pressure barrier around the strut members. The frame function is separate from the sealing pressure barrier.
Kojabashian (U.S. Pat. No. 3,880,500, 1975) described a frame design and a method utilizing two films that equalize the pneumatic forces on the supporting structure.
Soliday, et al. (U.S. Pat. No. 5,680,262 1997) utilizes a tubular frame of square cross section, and describes a tensioning method for the stretched membrane utilizing a plurality of pneumatic cylinders.
Carreras, et al (U.S. Pat. No. 6,332,687, 2001) describe utilizing a vacuum for primary deformation, and also using an outer mechanical ring and central plunger to achieve a more parabolic shape.
Each of these designs utilized some type of frame over which the reflective membrane is stretched:
Pajes (U.S. Pat. No. 2,952,189, 1960) and Bochmann (U.S. Pat. No. 3,610,738, 1971) utilized a drum design.
Kopitko (U.S. Pat. No. 3,031,928, 1962) utilized two parallel plates, with the film stretched at the midpoint.
One problem with previous designs is that although the reflective membrane affords an inexpensive method of presenting and manipulating large surfaces for the purpose of directing electromagnetic radiation or other energy, the framework for the membrane must be very strong to withstand the considerable pneumatic forces required to establish the membrane shape with minimal distortion. The primary aim of the frame is to establish a nearly perfect round shape with a circumferential raised portion that establishes a near perfect plane for the attachment and sealing of a non-porous membrane to the frame structure. An internal cavity is required to allow for the deformation of the membrane material. These past designs often utilized heavy and expensive frames.
Kopitko (U.S. Pat. No. 3,031,928, 1962), and Brantley, Jr. et al, (U.S. Pat. No. 4,033,676 1977), Kojabashian (U.S. Pat. No. 3,880,500,1975), Leonhardt, et al (U.S. Pat. No. 4,352,112 1982), and Soliday, et al. (U.S. Pat. No. 5,680,262 1997) all address this problem by utilizing two membranes with the vacuum or pressure within, thus equalizing much of the resulting forces. This has the disadvantage in that since both membranes stretch, the frame needs to be of considerable width to prevent the two membranes from touching in the middle when a vacuum is applied. Also, the sole structural element becomes the outer ring or drum, which can still oval and warp, distorting the desired focus or other manipulation of the directed electromagnetic or other radiation. The ovaling stress is non-uniform, and also tends to exacerbate the formation of wrinkles or waves in the membrane surface.
Inflatable systems eliminate much of the weight problem, and add a portability function, but tend to experience more distortion.
Mcreary (U.S. Pat. No. 3,056,131, 1962) circumvents the frame with an inflatable dish with reflective material on the back side, and transparent material at the front.
Wladimir von Maydell et al. (U.S. Pat. No. 3,326,624) describes an inflatable mirror for use in space-based applications.
One object of this invention that is believed to solve many of these problems is an improved frame design, utilizing a combination of two basic geometric shapes (a ring and a flat plane) which are mutually reinforced to arrive at a lightweight, rigid and true structure for supporting the stretched membrane.
Leonhardt, et al (U.S. Pat. No. 4,352,112 1982) describe utilizing a circumferential ring or rings with a vertical member to increase height in a drum-like arrangement. The ring or rings, however are described with two membrane surfaces forming the top and bottom walls of the structure.
Ring reinforcement has been applied in the past for a wide variety of unrelated applications, such as rolling a lip into the rim of a paper cup, a formed lip of a jar lid, and as with the molded lip on a container or with flying discs:
Schmidt (U.S. Pat. No. 4,130,234, 1978) and others discuss improved methods of rolling a lip surface into a cup design for greater strength. The goal here is to economically increase the strength of the rim.
Sassak (U.S. Pat. No. 5,116,275, 1992) discusses a flying toy with a reinforcing lip, but molded in.
Weiss (U.S. Pat. No. 5,366,403, 1994) discusses utilizing a reinforcing ring to convert a disposable plate into a flying toy.
Gilliam, et al. 6,761,283 2004 and others describe molding a lip, or “brow ridge” into a container and closure for same as a way of increasing the strength.
The primary function of the ring in this invention is unique—to provide mutual reinforcement along with a backplane element for the purpose of establishing a nearly perfect round shape with a circumferential raised portion that establishes a near perfect planar surface for the attachment and sealing of the membrane. Further, the application of negative or positive pressure inside the proposed structure causes a uniform warpage of the backplane element, increasing the strength of the structure without distortion of the manipulated electromagnetic, acoustic, or other energy.
Another object of this invention are methods of preventing wrinkles when a stretched membrane (esp. of polymeric material) is tensioned in multiple directions, as when it is stretched over a frame in a uniform manner, and then pneumatically deformed.
Sail battens have been utilized for minimizing wrinkling of sails, and also for shaping sails into an ideal aerodynamic form. Also similar to this invention, sail battens float on (are solely support by) the stiffened sail surface.
Mauney (U.S. Pat. No. 2,743,510 1956) describes an inflatable batten, and also one that forms an arc, but not a complete circle
Leonhardt, et al (U.S. Pat. No. 4,352,112 1982) describes prestressing a diaphragm in order to eliminate folds, and also discusses various reinforcing strips for the purpose of altering the stressed membrane configuration. The reinforcing ring depicted forms only an arc, and is not completely annular.
Henderson (U.S. Pat. No. 5,333,569 1994) describes an inflatable sail batten
Baird (U.S. Pat. No. 5,095,837 1992) describes a ram air inflatable form for a spinnaker sail, again with a curved surface, but not a completely round surface.
None of this prior art relates to preventing wrinkles in 360 degree round structure through the use of a circumferential batten.
Another object of this invention is to provide a means of uniformly tensioning the reflective membrane without inducing distortion via the application of heat to the circumference of the ring. Ambient temperature changes may affect the membrane material in a different way than the supporting structure, due to differences in thermal coefficient of expansion. The membrane may remain tightly stretched at one ambient temperature, but become loose when the ambient temperature is lowered, or vica versa. This problem increases as the size of the structure increases.
Martinez (U.S. Pat. Nos. 3,687,524, 3,757,479, and 3,877,139 1972-1975) described a method of tensioning a membrane mirror surface through the application of heat to shrink the film. This is a one-time manufacturing process, however, as opposed to a method of controlling the tension to accommodate changing ambient temperatures.
Sallis, (U.S. Pat. No. 4,741,609 1988) described a tensioning method for a membrane reflector utilizing an inflatable bladder, but no thermal component is mentioned.
Deppert, et al. (U.S. Pat. No. 4,987,826 1991) describes a dynamic thermal tensioning method for a piston rod-to-cylinder sealing ring, but utilizing a fluid filling medium.
Moore (U.S. Pat. No. 5,590,497 1997) discussed mechanical circumferential tensioning of prestress cables in a concrete tank.
Smith, et al. (U.S. Pat. No. 5,813,830 1998) utilizes springs for circumferential tensioning of a carbon seal containment barrier system inside a turbine engine.
Henderson, et al. (U.S. Pat. No. 5,990,851 1999) discusses circumferential tensioning of a space-based antennae, but not by thermal means.
Papadopoulas (U.S. Pat. No. 6,716,017 2004) discusses circumferential tensioning of an embossing roll, but not by thermal means.